(1) Field of the Invention
The present invention relates to an image formation device for a photocopier or similar, and particularly concerns technology for improving color image corrections.
(2) Description of the Related Art
Take, for example, a tandem color image formation device. In such a device, imaging units in each of a set of colors, each centered on a photosensitive drum and transfer equipment, are arranged along an intermediate transfer belt. Each of the imaging units forms a latent image on the photosensitive drum thereof upon exposure to a sweeping light beam from an exposure unit. After the formation of toner images developed through the application of toner to the latent images, the toner images are overlaid in a primary transfer to the intermediate transfer belt, followed by a secondary transfer at a secondary transfer position to a recording sheet passing between the intermediate transfer belt and an adjacent secondary transfer roller of each toner image into a single whole.
In such a color image formation device, misregistration can occur as a result of exposure position discrepancies (writing misalignment) in the light beam for each color. This problem is caused by mounting position changes that occur when the imaging units or the intermediate transfer unit are replaced, and by expansion or contraction of the photosensitive drums, intermediate transfer belt, and the casing for the imaging units that accompanies internal temperature changes.
This misregistration can be prevented by first having the imaging units in each of four colors, cyan (C), magenta (M), yellow (Y), and black (K), form reference patterns of a predetermined shape in each respective color on the intermediate transfer belt, then having an optical sensor take misalignment readings from the reference patterns in each color, and finally performing image writing position corrections according the misalignment readings so taken (overall process hereinafter referred to as misregistration corrections). Incidentally, forming reference patterns in all four colors for each misregistration correction process consumes a great deal of color toner and also requires an ample amount of processing time.
Japanese Patent Application Publication No. 2006-201339 discloses technology for suppressing toner consumption and reducing processing time needed for misregistration correction. This technology involves reducing the incidence of registration discrepancies in the photosensitive units (imaging units) of each color by driving the C, M, and Y units with a common motion lever as a single whole when pressing and separating the units onto and from the intermediate transfer belt. Therefore, predicated on the absence of registration discrepancies between the color (C, M, and Y) photosensitive units, register marks (registration patterns) can be formed in any one of these colors and in the K color such that misregistration corrections for the one color are also applicable to registration adjustments (misregistration corrections) made in the other colors.
Also, Japanese Patent Application Publication No. 2006-201341 discloses other technology in which the photosensitive units in each (C, M, and Y) color are unified into one, thus reducing the incidence of registration discrepancies between the color photosensitive units. Much as above, predicated on the absence of registration discrepancies between the color photosensitive units, register marks (registration patterns) can be fowled in any one of these colors (C, M, or Y) and in the K color such that misregistration corrections for the one color are also applicable to registration adjustments (misregistration corrections) made in the other colors.
However, according to the structures of Japanese Patent Application Publications No. 2006-201339 and 2006-201341, the unified color photosensitive units may yet undergo expansion or contraction due to internal temperature changes. Thus, misregistration caused by such temperature changes cannot be completely suppressed therein.
Nevertheless, using the misregistration correction reading from a register pattern formed in one of the three colors as-is for the other colors may lead to image quality degradation as appropriate misregistration corrections are not applied to the other colors.
In addition, the need to provide a motion lever or other mechanism combining the photosensitive units in order to minutely preserve the positions thereof contributes to cost escalation.